In wireless communication systems, the use of antenna arrays at the base station has been shown to increase both range, through increased gain, and capacity, through interference suppression. With adaptive antenna arrays, the signals received by multiple antenna elements are weighted and combined to improve system performance, e.g., by maximizing the desired receive signal power and/or suppressing interference. The performance of an adaptive antenna array increases dramatically with the number of antennas. Referring to an article entitled, "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems," by J. H. Winters, R. D. Gitlin and J. Salz, in IEEE Trans. on Communications, April 1994, it is shown that using an M element antenna array with optimum combining of the received signals can eliminate N.ltoreq.M-1 interferers and achieve an M-N fold diversity gain against multipath fading; .sup.- resulting in increased range.
Most base stations today, however, utilize only two receive antennas with suboptimum processing, e.g., selection diversity where the antenna having the larger signal power is selected for reception and processing. It is desirable to be able to modify existing base stations to accommodate larger arrays of antennas and/or improved received signal combining techniques. However, modifying existing equipment is difficult, time consuming, and costly, in particular since equipment currently in the field is from a variety of vendors.
One alternative is to utilize an applique, which is an outboard signal processing box, interposed between the current base antennas and the input to the base station, which adaptively weights and combines the received signals fed to the base station, optionally utilizing additional antennas. FIG. 1 shows a base station utilizing an applique. A key to the viability of utilizing the applique approach is that it should require little, if any, modification of the base station equipment. This constraint implies that the processing performed by the applique must be transparent to the existing equipment. Ideally, the signal emerging from the applique should appear to the existing base station as a high-quality received signal from a single antenna.
In practice, the received signals from the antennas are typically downconverted from radio frequency (RF) to baseband for subsequent processing by the adaptive array. Due to various implementation-related effects, especially when homodyne receivers are used, it is common for the resultant baseband signals to contain small undesirable DC or near-DC components. When the power of the undesirable DC or near DC components is comparable to the system noise power, the performance of the system is significantly degraded if the DC or near DC components are left uncompensated.
In an adaptive array system, limited DC compensation is available through the normal process of adaptive combining. Although DC offset is typically an artifact of the receiving process, it is equivalent to an additive carrier frequency interferer in the incoming passband signal. Therefore, the DC offset is amenable to cancellation like any other additive interferer.
However any additive interferer present in the input signal represents a constraint on the degrees of freedom available to the array, reducing the capability of the system to minimize the mean-squared error (MSE) such as by mitigating other interferers and noise. If the power of the DC offset is comparable to the system noise floor, then the effective size of the antenna array is reduced by one element, and its performance against fading and noise can be significantly reduced. This is a particularly severe problem when the number of antenna array elements is small and the loss of one element is significant.
A traditional way to compensate for DC offset is to estimate the DC component in each baseband signal and to subtract the DC estimate before the adaptive combining operation, thus providing the remainder of the receiver with a substantially DC free input signal. Unfortunately, the DC offset is not pure DC. The DC offset may be time varying, which can be rather rapid, thus the estimate would have to be made over a suitably small time scale. This DC estimation is not so straightforward because the desired signal component of the input signal itself contains short term DC components, much larger than the DC offset, which can swamp any DC estimate that is made over a relatively short time interval. Thus, applying the traditional DC compensation method can result in the introduction of a significant DC offset in a signal that had a relatively small or no DC offset.
Therefore, there is a need to provide DC compensation without loss of degrees of freedom and with minimal sensitivity to the DC component in the input signal.